US6859751B2 - Planar inertial measurement units based on gyros and accelerometers with a common structure - Google Patents
Planar inertial measurement units based on gyros and accelerometers with a common structure Download PDFInfo
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- US6859751B2 US6859751B2 US10/321,774 US32177402A US6859751B2 US 6859751 B2 US6859751 B2 US 6859751B2 US 32177402 A US32177402 A US 32177402A US 6859751 B2 US6859751 B2 US 6859751B2
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
- G01C19/00—Gyroscopes; Turn-sensitive devices using vibrating masses; Turn-sensitive devices without moving masses; Measuring angular rate using gyroscopic effects
- G01C19/56—Turn-sensitive devices using vibrating masses, e.g. vibratory angular rate sensors based on Coriolis forces
- G01C19/5719—Turn-sensitive devices using vibrating masses, e.g. vibratory angular rate sensors based on Coriolis forces using planar vibrating masses driven in a translation vibration along an axis
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
- G01C21/00—Navigation; Navigational instruments not provided for in groups G01C1/00 - G01C19/00
- G01C21/10—Navigation; Navigational instruments not provided for in groups G01C1/00 - G01C19/00 by using measurements of speed or acceleration
- G01C21/12—Navigation; Navigational instruments not provided for in groups G01C1/00 - G01C19/00 by using measurements of speed or acceleration executed aboard the object being navigated; Dead reckoning
- G01C21/16—Navigation; Navigational instruments not provided for in groups G01C1/00 - G01C19/00 by using measurements of speed or acceleration executed aboard the object being navigated; Dead reckoning by integrating acceleration or speed, i.e. inertial navigation
- G01C21/166—Mechanical, construction or arrangement details of inertial navigation systems
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
- G01C21/00—Navigation; Navigational instruments not provided for in groups G01C1/00 - G01C19/00
- G01C21/10—Navigation; Navigational instruments not provided for in groups G01C1/00 - G01C19/00 by using measurements of speed or acceleration
- G01C21/12—Navigation; Navigational instruments not provided for in groups G01C1/00 - G01C19/00 by using measurements of speed or acceleration executed aboard the object being navigated; Dead reckoning
- G01C21/16—Navigation; Navigational instruments not provided for in groups G01C1/00 - G01C19/00 by using measurements of speed or acceleration executed aboard the object being navigated; Dead reckoning by integrating acceleration or speed, i.e. inertial navigation
- G01C21/183—Compensation of inertial measurements, e.g. for temperature effects
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
- G01P15/00—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration
- G01P15/18—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration in two or more dimensions
Definitions
- IMUs Inertial Measurement Units
- MEMS Microelectromechanical Systems
- This invention relates to the design and fabrication of integrated, planar inertial measurement units (IMUs) based on planar gyroscopes and accelerometers having a common structure.
- the common structure is the key to a simpler structural design that is easier to build resulting in high fabrication yield without which integration is not possible.
- the common structure also simplifies the functional design resulting in improved performance.
- MEMS integration benefits performance because it prevents tolerance build-up that occurs with the assembly of separate parts to form the whole. With the absence of tolerance build-up, errors due to uncertainty are reduced and instrument stability improved.
- This invention also relates to gyroscope and accelerometer designs based on the common structure.
- a set of gyroscopes and accelerometers results that becomes the basis from which various IMUs can be designed depending on the application and performance requirements.
- instruments can also be structurally combined by sharing a common member to produce sets of instruments that are smaller, require less electronics and perform better.
- Two-instrument sets can be formed from which various IMUs can be designed.
- Three-instrument sets, four-instrument sets, etc. can also be formed from which IMU designs can be formed.
- an Inertial Measurement Unit can contain any number of gyroscopes and accelerometers.
- the typical IMU is a six degree-of-freedom (DOF) design containing three single DOF gyroscopes and three single DOF accelerometers.
- DOF degree-of-freedom
- some applications do not require the measurement of six degrees of freedom and therefore a lower number of instruments will suffice.
- Various combinations of gyroscopes and accelerometers will occur to those skilled in the art of IMU design.
- FIG. 1 is a stick figure of the conceptual common structure.
- FIGS. 2 a , 2 b , 2 c are conceptual renditions of three prospective planar gyroscope configurations possible from the common structure.
- FIG. 3 is a conceptual rendition of a planar gyroscope based on the second configuration ( FIG. 2 b ) mechanized with capacitive comb drives and comb pick-offs.
- FIG. 4 is a conceptual rendition of a planar gyroscope based on the third configuration ( FIG. 2 c ) mechanized with capacitive comb drives and comb pick-offs.
- FIG. 5 is a conceptual rendition of a planar gyroscope based on the second configuration with opposing capacitive plates for the drive and pick-offs.
- FIGS. 6 a , 6 b , 6 c are conceptual renditions of three prospective planar accelerometers possible from the common structure.
- FIG. 7 is a conceptual rendition of the second accelerometer configuration ( FIG. 6 b ) modified to enable dynamic tuning.
- FIG. 8 is a conceptual rendition for a multi-sensor based on the common structure.
- FIG. 9 a is a conceptual rendition of an integrated IMU based on planar gyroscopes and accelerometers having the common structure with a first accelerometer arrangement for measuring rotation rate about the axis normal to the plane.
- FIG. 9 b is a conceptual rendition of an integrated IMU based on planar gyroscopes and accelerometers having the common structure with a second accelerometer arrangement for measuring rotation rate about the axis normal to the plane.
- FIG. 9 c is a conceptual rendition of an integrated IMU based on planar gyroscopes of the third configuration.
- FIG. 10 is a conceptual rendition of a gyroscope and accelerometer instrument set that shares a common outer member.
- FIG. 11 is a conceptual rendition of a two accelerometer instrument set that shares a common drive member.
- FIG. 12 is a conceptual rendition of an integrated IMU based on instrument sets.
- FIG. 13 is a conceptual rendition of an integrated IMU based on instrument sets that are driven by one set of drive electrodes.
- FIG. 14 is a seven step, four mask process for fabricating the above devices.
- a six degree-of-freedom (DOF) IMU traditionally refers to three gyros and three accelerometers combined on a common member. The instruments are aligned to measure rotations about three orthogonal axes and accelerations along three orthogonal axes. Other six DOF IMU configurations are possible with a different combination of gyroscopes and accelerometers. Additional instruments may be added to improve performance.
- DOF degree-of-freedom
- This invention applies to the design and fabrication of a planar integrated IMU using principally MicroElectroMechanical Systems (MEMS) technology. Other planar technologies may be applicable.
- MEMS MicroElectroMechanical Systems
- Other planar technologies may be applicable.
- MEMS MicroElectroMechanical Systems
- the gyroscope and accelerometer designs are based on a common structure. The common structure can be described with common analysis. An IMU based on common instruments will reduce development time and risks associated with the development of different instruments.
- This invention also applies to the design and fabrication of the planar gyroscopes and accelerometers that are based on the common structure.
- the common structure 10 is illustrated in stick form in FIG. 1 . It comprises an inner member 11 that is flexurally connected to an outer member 12 that is in turn flexurally connected to the case 14 . Two sets of flexures define orthogonal axes of rotation for the inner and outer members, respectively.
- the outer member is driven into sinusoidal oscillation about the Drive Axis 15 .
- the inner member is the sense member that responds to gyroscopic torque by oscillating about the Output Axis 16 at the same frequency as the outer drive member, but at an amplitude that is proportional to rotation rate.
- an unbalance mass 17 is added to the inner member converting it into a pendulum 18 .
- the pendulum responds to acceleration by rotating about the Output Axis.
- the s,i,o co-ordinate frame rotates with the inner sense member about the Output Axis by angle .
- the a,b,c co-ordinate frame rotates inertial space with the case and vehicle (strap-down implementation).
- SDF single-degree-of-freedom planar gyroscope configurations are possible to consider from the common structure described by the stick figure of FIG. 1 . They are shown in FIGS. 2 a , 2 b , 2 c.
- the gyroscope configurations are distinguished by the orientations of the Drive Axis (i), Output Axis (o) and Input Axis (s) and the motion of the inner and outer members relative to the plane.
- the inner sense member 21 rotationally oscillates in and out of the plane about the Output Axis 22 by angle relative to the outer drive member 23 .
- the outer drive member is rotationally oscillated in and out of the plane about the Drive Axis 24 by angle ⁇ relative to the case 25 .
- the inner sense member is also driven relative to the case.
- the Drive and Output Axes are in the plane and orthogonal to each other.
- the Input Axis 26 is expected to be normal to the plane.
- a practical gyroscope however may be operated with a small offset between resonance frequencies.
- the outer member is driven at resonance to minimize power consumption and the inner member responds off-resonance at the outer member frequency.
- the resonance frequencies are set by the stiffness of the flexures and inertia of the members.
- the outer member oscillation amplitude is held constant so that the output per given rotation rate is constant (constant scale factor).
- FIG. 3 A gyroscope embodiment based on the second configuration 50 is illustrated in FIG. 3 .
- the inner sense member 51 is connected with four radial flexures 52 to the outer drive member 53 (ring shaped).
- the outer drive member is connected with a pair of torsional flexures 54 to the case 55 .
- Drive comb actuators 56 oscillate the ring in and out of the plane.
- Ring comb pick-offs 57 sense the oscillation amplitude of the ring about the Drive Axis 58 .
- Two sets of pick-offs are used that enable differential operation to eliminate common mode noise between them resulting in signals related to the motion only.
- Two sets of inner sense member comb pick-offs measure the output oscillation of the inner sense member relative to the outer member ring.
- the pick-offs are connected differentially.
- the comb drive and pick-off designs are shown conceptually. In reality a large number of comb fingers are used along the circumference of the members.
- a surface isolation film 65 is deposited over the torsional flexures to enable conductors to pass across them to separately connect to the two halves of the inner member.
- FIG. 4 A gyroscope embodiment based on the third configuration 70 is illustrated in FIG. 4 .
- the inner sense member 71 is connected with two torsional flexures 72 to the outer drive member 73 (ring shaped).
- the outer drive member is connected with four radial flexures 74 to the case 75 (at least three flexures are needed).
- Comb actuators 76 drive the ring to oscillate about the normal to the plane.
- Ring comb pick-offs 77 sense the oscillation amplitude of the ring about the Drive Axis 78 .
- Two sets of pick-offs enable differential operation to eliminate common mode noise between them, resulting in signals related to the motion only.
- Two sets of inner member comb pick-offs 79 measure the output oscillation of the inner sense member relative to the outer member ring.
- the pick-offs are connected differentially.
- the comb drive and pick-off designs are shown conceptually. In reality a large number of comb fingers are used along the circumference of the members.
- the gyroscope is divided into electrical regions a-g using electrical isolation spacers to enable the independent operation of the pick-offs and drives.
- Spacer 80 separates two halves of the inner sense member for differential operation of the pick-offs.
- Spacers 81 isolate the inner member from the outer member ring to enable operation of the pick-off.
- Spacers 82 , 83 isolate the comb drive from the ring to enable operation of the drive comb actuator.
- Spacers 83 , 84 isolate the outer member pick-off from the comb drives.
- a surface isolation film 85 is deposited over two radial flexures to enable conductors to pass across them to allow connection to the two halves of the inner member.
- FIG. 5 An embodiment of the gyroscope of the second configuration with opposing capacitive plates 90 instead of comb designs is illustrated in FIG. 5 .
- Two layers are used: a device layer 91 and a substrate layer 92 (Pyrex in this case).
- the device layer is bonded to the substrate via a mesa 93 in the device layer.
- the mesa forms the capacitive gap 94 and allows motion of the outer member 95 in and out of the plane.
- the mesa and the Pyrex Layer form the case 96 .
- One set of drive capacitive plates 97 oscillates the outer member relative to the case.
- a second set (not shown) is used to measure the outer member motion.
- a third set, pick-off plates 98 measures the motion of the inner sense member 99 .
- isolation spacers are not needed since the capacitive plates on the Pyrex Layer are isolated since Pyrex is an electrical insulator.
- a disadvantage of this design is that the sensed motion of the inner sense member pick-off contains the combined motions of the inner sense member relative to the outer member and relative to the case. A careful design of the inner sense member pick-off can reduce the sensed motion relative to the case, however. Surface isolation is not needed because connections are made on the Pyrex surface.
- An advantages of the two layer design is the flexibility to select materials and dimensions of one layer somewhat independently of the other.
- An opposing capacitor plate design can be carried out where the substrate layer is of the same material as the device layer to reduce bimetallic stress.
- the substrate layer can be made of a different thickness to add stability.
- Combs and plates can be combined to actuate and sense motion of the instrument members.
- the combination used depends on the functionality desired.
- the two resonance frequencies of the fabricated gyro may not be matched, it may be necessary to include a mechanism in the design that will allow tuning after fabrication.
- the approach is to shape the inner member of the gyro of the second configuration so that it has a tuning inertia like the accelerometer. After the outer member is driven to resonance, the amplitude of the outer member oscillation is varied to tune the flexure stiffness of the inner member relative to the resonance frequency of the outer member.
- the accelerometer based on the common structure comprises a pendulum that is oscillated about the long axis of the pendulum through the use of the outer member.
- the oscillation dynamically tunes the stiffness of the flexures of the inner member so as to make them effectively weaker for motion of the pendulum about the Output Axis.
- the main benefits derived are increased accelerometer sensitivity and reduced bias instability related to the pick-off instability.
- the accelerometer is disclosed in U.S. Pat. No. 6,338,274, incorporated by reference herein.
- the dynamically tuned accelerometer is obtained conceptually from the common design by adding mass to the inner sense members of the gyroscope configurations to make them pendulous as shown in FIGS. 6 a , 6 b and 6 c.
- the three DTA configurations are distinguished by the Drive Axis (i), the Output Axis (o) and the accelerometer Input Axis.
- the inner member 101 rotates out of the plane due to the action of acceleration on the pendulous mass 105 .
- the outer member 102 is oscillated in and out of the plane about the Drive Axis 103 .
- the accelerometer Input Axis 104 is normal to the plane.
- the inner member 101 rotates in the plane about the Output Axis 102 and the outer member 103 oscillates about an axis in the plane.
- the location of the pendulous mass 115 determines the direction of the Input Axis.
- the accelerometer Input Axis 104 and the Drive Axis are in the plane.
- the inner member 121 rotates about the Output Axis 122 in the plane and the outer member 123 oscillates about the Drive Axis 124 normal to the plane.
- the accelerometer Input Axis 125 is normal to the plane and aligned with the Drive Axis.
- the bracketed term contains the sum of the flexure stiffness and the dynamic stiffness.
- I i ,I s are inertias about the i-axis and s-axis, respectively.
- a tunable second configuration accelerometer 130 comprises an inner sense member 131 that is not symmetric about the i-axis and s-axis. Radial flexures 132 enable the rotation of the pendulum in the plane.
- the accelerometer is also operated closed loop otherwise an effectively weakened flexure will result in bottoming of the pendulum against stops.
- the accelerometer Since the accelerometer is based on the same structure as the gyroscope it will have some sensitivity to rotation rate depending on how well it meets the conditions for operation of the gyroscope. Two conditions prevent the accelerometer from being a gyroscope however: the inner member resonance frequency is designed to be much lower than the outer drive member resonance frequency and the oscillation output of the inner sense member is filtered since it is sinusoidal.
- the common structure can be mechanized to form the gyro or the accelerometer as discussed above.
- the multi-sensor embodiment 220 is shown in FIG. 8 . It combines the gyroscope second configuration with the tunable accelerometer second configuration.
- the key feature is an inner member 221 with the appropriate tuning inertia.
- Other conditions include the proper choice of flexure stiffness and inertias for the inner and outer members 222 .
- the gyroscope function operates as the gyroscope of the second configuration but with tunability.
- the accelerometer operates as the accelerometer of the second configuration.
- the gyroscope and accelerometer outputs are separable because the gyroscope output is oscillatory and the accelerometer output is DC level.
- the integrated IMUs are planar embodiments comprising various distributions of planar gyroscopes and accelerometers having the common structure, as described above. That is, each gyroscope and accelerometer is composed of an outer member that is driven and an inner sense member that responds to either rotation rate or acceleration.
- a first six DOF, integrated IMU embodiment 140 is based on gyroscopes of the second configuration and accelerometers of the first and second configurations as shown in FIG. 9 a. These configurations are further distinguished by outer drive members that oscillate in and out of the plane.
- the x, y, z axes form the co-ordinate frame for the IMU.
- the x-axis and y-axis are in the plane and the z-axis is normal to the plane.
- Two gyroscopes of the second configuration are used to sense rotation rates about the x-axis and y-axis by aligning the Input Axis of the first with the x-axis and aligning the Input Axis of the second with the y-axis.
- the accelerometer of the first configuration is used to sense acceleration along the z-axis since its Input Axis is normal to the plane. It is identified as A z .
- Two accelerometers of the second configuration are used to sense acceleration input along the x-axis and y-axis. These are identified as A x and A y .
- each instrument is designed to occupy a square space (cell) of the same size. This allows any orientation or location of instruments in the IMU design.
- the first IMU comprises nine unit cells arranged in a three by three matrix.
- In the first row from left to right, are located gyro G y 141 , accelerometer A y 142 and gyro G x 143 .
- In the second row are located accelerometer A x 144 , a space left for test devices and a second accelerometer A x 145 .
- In the third row are located gyro G x 146 , accelerometer A y 147 and accelerometer A z 148 .
- Each instrument is attached to the substrate 149 by mesa structures 150 or similar structures that support the devices from the substrate so the devices are free to move.
- the purpose of this arrangement of instruments is to place four accelerometers, two A x and two A y in a cross configuration 151 , with the accelerometers displaced an equal distance R from the IMU center.
- the cross configuration of accelerometers responds to the sum of linear and centrifugal accelerations.
- rotation rate and linear accelerations along the two axes in the plane can be separated.
- the separation of signals is possible because rotation rate causes all the pendulums to rotate outwards while acceleration causes one pendulum to rotate outwards and the second to rotate inwards for each set of two accelerometers along each axis.
- a second benefit of the cross configuration of accelerometers is that the measurement of acceleration along the x-axis and along the y-axis can be done differentially by each set of two accelerometers. Differential operation is a means to cancel non-acceleration, common mode signals.
- the two y-gyroscopes can also be operated differentially.
- a first variation on the first embodiment can be obtained by adding a second gyro G y in the location of accelerometer A z and moving the accelerometer into the center cell position. The result is that all instruments are placed symmetrically about the center of the IMU and all except for the z accelerometer can be operated differentially.
- a second variation on the first embodiment is to use any other planar accelerometer design regardless of whether it is dynamically tunable or not.
- the second IMU embodiment 160 comprises nine unit cells arranged in a three by three matrix.
- gyro G y 161 In the first row, from left to right, are located gyro G y 161 , accelerometer A x 162 and gyro G x 163 .
- accelerometer A y 164 In the second row are located accelerometer A y 164 , a space left for test devices and a second accelerometer A y 165 .
- In the third row are located gyro G x 166 , accelerometer A x 167 and accelerometer A z 168 .
- a variation on this embodiment places a second gyro G y at the location of accelerometer A z and moves the accelerometer to the central cell position.
- Two gyroscopes of the third configuration are used to sense rotation rates about the x-axis and y-axis by aligning the Input Axis of the first with the x-axis and aligning the Input Axis of the second with the y-axis. These gyroscopes are identified as G x and G y .
- the accelerometer of the first configuration is used to sense acceleration along the z-axis since its Input Axis is normal to the plane. It is identified as A z .
- Two accelerometers of the second configuration are used to sense acceleration input along the x-axis and y-axis. These are identified as A x and A y .
- each instrument can be designed to occupy a square space (cell) of the same size. This allows any orientation or location of instruments in the IMU design.
- the cross configuration of the accelerometers is the same as it was for the first IMU embodiment. It senses the sum of the linear and centrifugal accelerations. The signals are separated as described for the first embodiment.
- the third IMU embodiment 170 comprises nine unit cells arranged in a three by three matrix.
- gyro G y 171 In the first row, from left to right, are located gyro G y 171 , accelerometer A y 172 and gyro G x 173 .
- accelerometer A x 174 In the second row are located accelerometer A x 174 , a space left for test devices and a second accelerometer A x 175 .
- In the third row are located gyro G x 176 , accelerometer A y 177 and accelerometer A z 178 .
- the first embodiment of an instrument set 180 combines gyro G x 181 with accelerometer A x 182 or equivalently gyro G y with accelerometer A y as shown in FIG. 10.
- a capacitive comb drive 183 is used to oscillate the outer member 184 in and out of the plane. Separate pick-offs (not shown) are used to sense the motion of the inner members.
- the outer member is connected with torsional flexures 185 to the case mesa 186 .
- the significance of the gyro/ accelerometer set is that the gyro sensitivity of the accelerometer to rotation is about the same axis as the gyroscope, therefore the gyro signal can be used to compensate the accelerometer for its gyro-related error.
- a consideration for the design of the gyro/accelerometer instrument set is that the requirements for the gyroscope and accelerometer need to be met separately.
- FIG. 9 a A close inspection of FIG. 9 a indicates that a second level of integration is possible if instrument sets are used.
- the arrangement of the instrument sets in the IMU embodiment 200 is shown in FIG. 12 .
- Three gyro/accelerometer sets 201 and one accelerometer/accelerometer set 202 are used.
- Four accelerometers are arranged in a cross configuration 203 as before to enable the sense of rotation rate in addition to linear acceleration.
- the centrifugal acceleration mode is used.
- a z-gyro can be added to the center cell to form a variation on the IMU embodiment.
- a variation on the IMU embodiment uses the instrument sets so that the drive axes of all four sets are arranged radially along the x-axis and y-axis.
- the four accelerometer configuration measures angular acceleration plus linear acceleration.
- An additional level of integration or perhaps simplification can be made to the embodiment described in FIG. 12 by using common drive electrodes 207 , 208 to drive all the outer members 209 to form the embodiment 210 shown in FIG. 13.
- a consideration for this design is the drive of the four outer members at the same resonance frequency. Resonance drive is used to minimize the power needed. Tuning mechanisms may be needed for the outer members.
- a benefit of this design includes one set of drive electronics for the IMU.
- the fabrication approach is described for one instrument but applies equally as well to a set of instruments fabricated at the same time on one substrate.
- the Dissolved Wafer Process (DWP) is representative of a typical process. The process is a good match to the requirements for the integrated IMU design that include the use of one structural material for the devices to reduce stress and warpage on the parts and to allow the fabrication of instruments on a common substrate that are separated structurally at the device level.
- the DWP process is representative of a transfer process in that the devices are fabricated on one substrate and transferred onto a second substrate.
- the advantage of the DWP is the flexibility in the choice of materials and thickness of the device and substrate layers. Low stress devices can be fabricated onto a stable rigid base layer for stability.
- the process flow is described using five masks in FIG. 14 .
- the resulting two-wafer lamination is cut to separate the IMUs.
- the individual IMUs are placed into chambers of a Teflon container and all immersed into a chemical EDP etch to dissolve the silicon substrate 241 leaving untouched the epitaxial structures 242 bonded to the Pyrex 243 .
- the epitaxial structure comprises the set of devices.
- the IMU chips are then placed into packages and wire bonded.
Abstract
Description
- Consideration1—MEMS integration is essential to achieving the best performance in a miniature IMU.
- Consideration2—MEMS gyroscopes and accelerometers based on a common structure reduce the requirements on fabrication processes thereby improving yield without which an Integrated IMU is not possible.
- Consideration3—MEMS gyroscopes and accelerometers based on the common structure simplify the IMU design.
- Consideration4—Standard gyroscope and accelerometer designs form the basis for designing various IMUs.
- Consideration5—An integrated IMU makes the most efficient use of space resulting in the smallest size.
- Consideration6—An integrated IMU requires one package that provides a common environment for all instruments.
where
- ISo inner sense member moment of inertia about the o-axis (Output Axis).
- DS inner sense member damping.
- KS inner sense member flexure stiffness.
- rotation angle of the inner sense member relative to the drive member
- {tilde over (φ)} rotation angle amplitude of the drive member relative to the case.
- Ωa,Ωb,Ωc rotation rates of the case in inertial space about three orthogonal axes.
- ΔIS=ISi−ISs inertia difference of the inner sense member inertias about the i-axis and s-axis (tuning inertia).
To the left of the equals sign in Equation (1) are included the usual torque terms dependent on inertia, damping and stiffness as well as a nonlinear term dependent on inner sense member angle of rotation. The stiffness term is dependent on vehicle rotation rates Ωa,Ωb,Ωc, outer member oscillation frequency ω and a factor referred to as the tuning inertia, ΔIS as shown in the following equation.
- ISoΩa{tilde over (φ)}ω cos ωt gyro torque for case rotation about the a-axis.
- ΔIS(ΩaΩb+ΩbΩc{tilde over (φ)} sin ωt+Ωa{tilde over (φ)}ω cos ωt) torque related to the inner sense member tuning inertia.
- τrebalance rebalance torque to maintain the inner sense member at null.
- τp=Pa pendulous torque applied to the inner sense member by acceleration, a, acting on pendulosity, P.
Gyroscope Mechanization
I So +D S +K S−(ΩaΩb+ΩaΩc{tilde over (φ)} sin ωt+Ω a{tilde over (φ)}ω cos ωt)2 =I SoΩa{tilde over (φ)}ω cos ωt (3)
where the non-linear term in 2 is further ignored because the output angle is typically very small. The second configuration enables a good strap-down gyro. To obtain maximum gyroscope response and therefore maximum oscillation amplitude of the inner sense member, Max, the inner and outer member resonance frequencies are matched.
a centrifugal =Ω 2 R (7)
- Step a: The starting material is a 4″ diameter silicon wafer 230 with a thickness of p++ boron diffused
Epitaxial layer 231 grown on top. - Step b:
Apply Mask 1 topattern mesas 232,posts 233 andwall structures 234; plasma etch into the epitaxial layer to form them with some height. The mesa is the point of contact between the silicon wafer and the Pyrex substrate after bonding. The mesa height is selected to form the capacitive gap that allows movement of the inner and outer members. The post electrically grounds the metallizations during anodic bonding so that the voltage applied does not destroy the metallizations. The wall is formed around the full IMU. Its function is to keep the gap volume clean during cutting of the bonded wafers. - Step c: Apply Mask 2 to pattern the accelerometer structure 235: plasma etch into the epitaxial layer to form the sidewalls of the device. Etch through the full thickness of the epitaxial layer and partially into the silicon substrate.
- Step d: Start the
Pyrex wafer 236. Apply Mask 3 to pattern a well 237 into the Pyrex. Plasma etch to form the well. The thickness of the mesa plus the well depth make up the gap. A gap dimension is selected that prevents sticking of the device members to the Pyrex that gives good capacitive sensing and actuation. - Step e: Apply Mask 4 to pattern recessed
trenches 238 in the Pyrex in preparation for metallization. Etch trenches into the glass to a suitable depth. - Step f: Deposit chromium/platinum metal film over the full wafer surface. This will deposit metal into the etched trenches formed in the previous step, extending slightly above the top of the trench. Remove the remaining resist and other metal using lift off, leaving only metal in the
trenches 239. - Step g: Anodically bond the Pyrex layer to the epitaxial side of the silicon wafer 240. The devices are ready for post processing.
Post Processing
Claims (49)
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US10/321,774 US6859751B2 (en) | 2001-12-17 | 2002-12-17 | Planar inertial measurement units based on gyros and accelerometers with a common structure |
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US34131201P | 2001-12-17 | 2001-12-17 | |
US10/321,774 US6859751B2 (en) | 2001-12-17 | 2002-12-17 | Planar inertial measurement units based on gyros and accelerometers with a common structure |
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